The disposal of waste water treatment sludge has become an increasingly difficult problem. In the past, the usual solution to the problem has been to dump the sludge on land or at sea. However, with the present increasing emphasis on improved environmental quality standards, dumping is no longer a viable solution to the problem. Moreover, due to stricter water quality standards, the amount of waste water treatment sludge to be disposed of has been continuously increasing while the land and sea areas available for dumping such sludge has been continuously decreasing.
The foregoing factors have necessitated the development of alternative methods of disposing of waste water treatment sludge. One such alternative method which has become attractive from environmental and economic standpoint is incineration of the sludge. Incineration reduces the sludge to a low volume inert ash which may be easily handled and disposed of. Generally, after an initial filtration to concentrate the sludge, incineration requires that the sludge be dried and combusted in some type of furnace, and frequently requires an external source of heat energy, such as the energy produced by the combustion of a common fuel. Various types of furnaces and other heating equipment have been used for incinerating sludge, such as multiple hearth furnaces, fluidized bed furnaces, travelling gate furnaces, atomized spray dryers and other heating equipment.
Another problem closely related to the disposal of waste water treatment sludge is the problem of treating slurries produced by various industrial processes. As with waste water treatment sludge, such slurries usually contain a relatively high percentage of water with varying percentages of organic and inorganic solid materials. Many such slurries contain valuable particulate solid materials which could be reused if recovered. Examples of this phenomena exist in the steel, aluminum, paper, chemical and petrochemical industries. One specific example is with respect to the process of extracting aluminum from its ore. In this process cryolite, a mineral consisting of sodium-aluminum floride, is used to form a molten bath in an electric reduction furnace. Small amounts of cryolite particles are entrained in the gases evolved from the reduction reaction and are collected in a gas cleaning system after such particles leave the furnace. In addition to cryolite particles, the evolved gases contain carbon particles and various organic materials. After collection in the gas cleaning system, the cryolite and carbon particles and organic materials are mixed with water to form a slurry. The slurrry is dried, and the carbon particles and organic materials are combusted in a multiple hearth furnace. The regenerated cryolite is taken from the furnace and returned as fresh makeup to the electric reduction furnace. This cryolite regeneration process and other similar processes in the above-listed industries usually are borderline from an economic standpoint and require the most efficient utilization of energy possible.